Composite materials based on epoxy resins have been used in a variety of day-to-day applications for a long time and continue to have considerable importance because of their versatility. A specific example of such an application includes but is not limited to electrical laminates used in printed circuit boards (printed wiring boards, PWB). The epoxy resins used therein have particularly gained popularity because of their ease of processibility. Those epoxy resins also feature good mechanical and chemical properties, such as for example, toughness and resistance to a variety of organic solvents and also display good chemical and moisture resistance. These properties permit the epoxy resin materials to be adapted to diverse application purposes and allow the materials sharing in the composite to be used advantageously.
Generally, the epoxy resins are readily processed into composite materials for PWB applications via the manufacturing of prepregs (B-staging). For example, the substrate material, which is typically an inorganic or organic reinforcing agent in the form of fibers, fleece and fabric or textile materials, is impregnated with the resin. This may be accomplished by coating the substrate with a resin solution in an easily vaporizable or volatilizable solvent. The coating may be carried out by a variety of well-known techniques including rolling, dipping, spraying, and combinations thereof. The prepregs are then heated in an oven chamber to remove solvent and to partially cure the resin. The prepregs obtained after this process must not self-adhere, but they also should not be fully cured. In addition, the prepregs must be sufficiently stable in storage. In the subsequent processing into composite materials, the prepregs must furthermore fuse when there is a rise in temperature and pressure and must bind together under pressure with the reinforcing agents or insertion components as well as with the materials provided for the composite as compactly and permanently as possible; that is the cross-linked epoxy resin matrix must form a high degree of interfacial adherence with the reinforcing agents, as well as with the materials to be bonded together, such as metallic, ceramic, mineral and organic materials.
A key requirement in many applications is the requirement for flame resistance. In many areas, this requirement is given first priority, due to the danger to human beings and material assets, for example in structural materials for airplane and motor vehicle construction and for public transportation vehicles. In electrotechnical and particularly electronic applications, it is absolutely necessary for the electrical laminate materials to be flame resistant, due to the substantial worth of the electronic components assembled thereon and the intrinsic value of human life associated with working on or near devices containing PWB components.
Accordingly, it has been customary in the preparation of epoxy-containing laminates to incorporate into the epoxy resin compositions various additives and/or reactives to improve the flame retardancy of the resulting laminate. Many types of flame retardant substances have been used, however, the most common thus far used commercially have been halogen containing compounds such as tetrabromobisphenol A. This material is typically incorporated into an epoxy resin by reaction with the diglycidyl ether of bisphenol A. Typically, in order to reach the desired fire retardancy level (V-0 in the standard “Underwriters Laboratory” test method UL 94), levels of such bromine-containing flame retardant substances are required that provide a bromine content from 10 weight percent to 25 weight percent based on the total weight in the product.
Generally, halogen-containing fire retardant epoxy resins such as those containing tetrabromobisphenol A are considered to be safe and effective. However, there has been increasing interest by some to utilize flame-retarded epoxy systems that are not based on halogen chemistry. It is desirable for these new materials to be able to meet the requirements of fire retardancy and to display the same advantages of mechanical properties, toughness, and solvent and moisture resistance that is offered with the halogenated materials currently used.
One such approach proposed by many researchers has been the use of phosphorus based fire retardants. See for example, EP 0 384 939; EP 0 384 940; EP 0 408 990; DE 4 308 185; DE 4 308 187; WO 96/07685; WO 96/07686; U.S. Pat. No. 5,648,171; U.S. Pat. No. 5,587,243; U.S. Pat. No. 5,576,357; U.S. Pat. No. 5,458,978; and U.S. Pat. No. 5,376,453; all of which are incorporated herein by reference in their entirety. In all of these references, a formulation is formed from the reaction of a flame retardant derived from a phosphorus compound and an epoxy resin, which is then cured with an amino cross-linker such as dicyandiamide, sulfanilamide, or some other nitrogen element containing cross-linker to form the thermosetting polymer network.
Specific examples of commercially available phosphorus-based fire retardant additives include Antiblaze™ 1045 (Albright and Wilson Ltd, United Kingdom) which is a phosphonic acid ester. Phosphoric acid esters have also been used as additives, such as, for example, PX-200 (Diahachi, Japan). Commercially available reactive phosphorus containing compounds that have been disclosed as being suitable for epoxy resins include Sanko HCA and Sanko HCA-HQ (Sanko Chemical Co., Ltd., Japan).
Alkyl and aryl substituted phosphonic acid esters have also been used to flame retard epoxy resins. More particularly, C1–C4 alkyl esters of phosphonic acid are of value because they contain a high proportion of phosphorus, and are thus able to impart fire retardant properties upon resins in which they are incorporated. However, the phosphonic acid esters are not satisfactory as a substitute for halogenated flame retardants in epoxy resins for the production of electrical laminates for various reasons. First and foremost of these reasons are the phosphonic acid esters often times impart undesirable properties. For example, the phosphonic acid esters are known plasticizers and thus the laminates formed therefrom tend to exhibit undesirable low glass transition temperatures (Tg). An additional drawback is that the use of phosphonic acid esters in amounts sufficient to provide the necessary flame retardancy increases the tendency of the resulting cured epoxy resin to absorb moisture. The moisture absorbency of the cured laminate board is very significant, because laminates containing high levels of moisture tend to blister and fail, when introduced to a bath of liquid solder at temperatures around 260° C., a typical step in the manufacture of printed wiring boards.
Various other phosphorus based flame retardant materials are described in the literature, which are either too expensive or feature certain inferior properties. For example, EP 0 754 728 discloses a cyclic phosphonate as a flame retardant material, which is incorporated into an epoxy resin. However, EP 0 754 728 indicates that this cyclic phosphonate should be present in large quantities, such as in excess of 18 weight percent, in order for the resin system to meet UL 94 V-0. This loading for a phosphonate compound may lead to a depression of the Tg or higher moisture absorption. EP 1 116 774 utilizes a hydrogen phosphinate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, in conjunction with triphenylphosphine oxide. However, the epoxy resin base requires the use of non-standard epoxy resins; namely a xylene-modified novolac resin and naphthylene aralkyl and biphenyl-modified epoxy resins. WO 99/00451 discloses another flame retardant composition utilizing phosphonic acid esters. Although this composition appears to exhibit improved flame retardant properties at low levels of phosphonic acid ester, there is still a need in the industry for a flame retardant epoxy resin with improved Tg and flame retardant properties. Phosphorus flame retardant additives, in general, can lead to a significant plastisizing effect (U.S. Pat. No. 5,587,243 and references cited therein). Also, in the case of additive compounds, there may be a question of the additives leaching from a thermoset polymer network under processing conditions or over time.
Other methods to impart flame retardancy involve preparation of halogen-free flame retardant epoxy resin compositions using a combination of resinous materials and an inorganic filler, such as aluminum trihydrate (EP 0 795 570 A1) or magnesium hydroxide (JP 2001213980 A2). These materials may, depending on the physical properties, render the processing of the epoxy resins more difficult, as they are insoluble in the resin systems. Additionally, fairly large load levels can be required, which can detract from the properties. See, generally, U.S. Pat. No. 6,097,100 and references cited therein for a description of various inorganic fillers and WO 01/42359.
Phosphorous compounds containing a high number of P-C bonds with a low or no number of P-O bonds have been recognized as having improved hydrolytic and thermal stability properties relative to the materials containing more P-O bonds. These types of phosphorus compounds have also been used to prepare halogen-free flame retardant epoxy resins useful in the manufacture of composite materials. For example, the use of phosphorus-carbon bonded moieties, such as phosphine oxides, have been disclosed in WO 01/42253; U.S. Pat. No. 4,345,059; EP 1 116 774; and JP2000186186, all of which are incorporated herein by reference in their entirety. A key disadvantage of these compositions, however, is that they are costly to prepare, because they utilize unique raw materials. For example, JP2000186186 discloses the use of pure bis(p-hydroxyphenyl)phenyl-phosphine oxide, which requires the use of a pure dichlorophenyl phosphine. In an analogous manner, the phosphine oxides utilized in WO 01/42253 require lithium reagents and cryogenic reaction conditions, thus warranting special equipment for its manufacture. The phosphine oxides display benefits of improved resistance to moisture uptake when compared with other phosphorus compounds that contain P-O bonded moieties, as disclosed in WO 01/42253. See also, U.S. Pat. No. 6,403,220 of Brennan et al.
Thus, it is an object of this invention to provide economical yet useful hydroxyarylphosphine oxide/phenolic compositions for curing epoxy resins having utility in the manufacture of composite materials, such as electrical laminates for printed wiring boards or printed circuit boards.
It is yet an other object of this invention to provide hydroxyarylphosphine oxide containing, hydrolytically and thermally stable, non-halogenated, flame resistant novolac resin mixtures, which are used as curing agents for epoxy resins for making laminates for printed wiring boards and various other composite materials.
Further, it is also an object of this invention to provide hydroxyaryl phosphine oxide/phenolic resin mixtures for curing epoxy resin compositions having improved flammability properties at reduced phosphorous content when compared with phosphine oxide cured epoxy laminates.
A still yet further object of this invention to provide halogen free phosphine oxide/novolac resin containing epoxy resin compositions that are useful as replacements for tetrabromobisphenol A in FR-4 laminate applications.
These and other objects and advantages of the invention will be seen from the following detailed description.